linux-ext4 - Re: SLAB_TYPESAFE_BY_RCU without constructors (was Re: [PATCH v4 13/17] khwasan: add hooks implementation)

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Message-ID: <20180731170957.o4vhopmzgedpo5sh@breakpoint.cc>
Date:   Tue, 31 Jul 2018 19:09:57 +0200
From:   Florian Westphal <fw@...len.de>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.com>,
        linux-ext4@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev@...r.kernel.org, Gerrit Renker <gerrit@....abdn.ac.uk>,
        dccp@...r.kernel.org, Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        David Airlie <airlied@...ux.ie>,
        intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        Eric Dumazet <edumazet@...gle.com>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Ursula Braun <ubraun@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: SLAB_TYPESAFE_BY_RCU without constructors (was Re: [PATCH v4
 13/17] khwasan: add hooks implementation)

Andrey Ryabinin <aryabinin@...tuozzo.com> wrote:
> Guys, it seems that we have a lot of code using SLAB_TYPESAFE_BY_RCU cache without constructor.
> I think it's nearly impossible to use that combination without having bugs.
> It's either you don't really need the SLAB_TYPESAFE_BY_RCU, or you need to have a constructor in kmem_cache.
> 
> Could you guys, please, verify your code if it's really need SLAB_TYPSAFE or constructor?
> 
> E.g. the netlink code look extremely suspicious:
> 
> 	/*
> 	 * Do not use kmem_cache_zalloc(), as this cache uses
> 	 * SLAB_TYPESAFE_BY_RCU.
> 	 */
> 	ct = kmem_cache_alloc(nf_conntrack_cachep, gfp);
> 	if (ct == NULL)
> 		goto out;
> 
> 	spin_lock_init(&ct->lock);
> 
> If nf_conntrack_cachep objects really used in rcu typesafe manner, than 'ct' returned by kmem_cache_alloc might still be
> in use by another cpu. So we just reinitialize spin_lock used by someone else?

That would be a bug, nf_conn objects are reference counted.

spinlock can only be used after object had its refcount incremented.

lookup operation on nf_conn object:
1. compare keys
2. attempt to obtain refcount (using _not_zero version)
3. compare keys again after refcount was obtained

if any of that fails, nf_conn candidate is skipped.